Photosensitive or photoreactive polymers are commonly used in coatings which have properties that can change when exposed to suitable radiation. Such a change in properties is usually a change in solubility and results in a sharp distinction between exposed and unexposed areas. Photocross-linking and photoinitiated polymerization are commonly used to exploit the photosensitivity of materials. This invention relates to photocross-linking between ethylenically unsaturated sites. Exposure to light of the photosensitive or photoreactive material results in a solubility change and creation of image in the exposed material. Most commonly, resist images are produced by solvent development of photoinduced cross-linking (negative processing). Photoresist polymer compositions must satisfy a number of physical property dependent requirements under practical working conditions. The most important, and most difficult, requirements to fulfill are image discrimination, thermal stability, ink resistance, and etch resistance when exposed. For image discrimination, the developing solvent must remove the soluble portion of the exposed material without distorting or swelling the insoluble areas to give a sharp, true image.
Exposure of the photopolymer to light generally provides energy necessary to cross-link adjacent polymeric molecules. In many instances, this provides for a three-dimensional cross-linked polymer. For example, with sufficient exposure to light, a polymer having diazo units becomes hydrophobic. A variety of photo-chemical systems have been used in photoresist applications including cinnamates, chalcones, p-azidophenyls, azidophthalates, p-phenylene, bis(acrylates), and many others.
One class of photosensitive compounds photoresist applications is based on photoinitiated dimerization involving a styryl pendant group, e.g. styryl pyridine. Early work with this class was performed by Douglas G. Borden, Jack L. R. Williams and others at Eastman Kodak, and it is discussed in Borden et al., "Photosensitive Polymers", Chemical Abstracts, Vol. 73. p. 10 (1970); Borden et al., "Light-Sensitive Polymers", Radiation Chemistry and Photochemistry, Vol. 72, p. 395 (1970); Borden et al., Photopolymer Design: Photocrosslinkable Styrylpyridinium Substituted Vinyl Polymers with Absorption Maxima from 270 nm to 540 nm, Makromolekulare Chemi, Vol. 178, pp. 3035-3049 (1977); Williams, "Light-Sensitive Polymeric Quaternary Salts", Williams, "Photodimerization of 2-styrylpyridine", The Journal of Organic Chemistry, Vol. 25, No. 11, pp. 1839-1840 (1960); Williams et al., "Cis and Trans Isomers of 2-styrylpyridine", The Journal of Organic Chemistry, Vol. 26, pp. 4893-4895 (1961); Leubner et al., U.S. Pat. No. 2,811,510; and Williams, U.S. Pat. No. 2,908,667. This work generally relates to the preparation of polymers having pendant styryl pyridinium functionality. The general preparation of the polymers is set out below: ##STR2## However, the Kodak technology generally required a "special approach" in their synthesis and, more importantly, final purification of the polymer.
Additional work in this area was performed by K. Ichimura and others at the Japanese Research Institute for Polymers and Textiles and is discussed in references including K. Ichimura, J. Poly. Sci. 20, 1411, 1982; K. Ichimura, et al., J. Poly. Sci. 20, 1419, 1982; Ichimura, et al., U.S. Pat. No. 4,777,144 (1988); U.S. Pat. No. 4,564,580 (1986); European Patent No. 130 804 (1984); Japanese Patent No. 63,/198045 (1988); etc. This technology involves using an acetal-forming reaction to graft a photodimerizable group onto a polyvinyl alcohol backbone as shown in a cross-linked and uncross-linked form. ##STR3## Unfortunately, this technology generally requires specific starting materials and appears to be limited to poly(vinyl alcohol) polymers.
Other water sensitive or water soluble polymeric materials are disclosed in Mukunoki et al., U.S. Pat. No. 4,917,993, which discloses the use of a particular class of water soluble synthetic or natural polymers in silver halide photographic materials. The synthetic polymers disclosed in Mukunoki preferably have nonionic and/or anionic groups appended onto a poly(methyl) backbone which may or may not be otherwise substituted. These appended groups are linked to the poly(methyl) backbone through the following oxygen-containing linking groups: --CONH--, --NHCO--, --COO--, --OCO--, or --O--.
Hamilton, et al., U.S. Pat. No. 5,061,603, discloses photopolymerizable polyvinyl alcohols having a pendant group of the formula: ##STR4## wherein R.sub.1 -R.sub.3 represent various substituents or hydrogen atoms. A particularly preferred R.sub.1 is a methyl group. The position disclosed for grafting this group to a polyvinyl alcohol backbone is disclosed as the 4-position of the benzene ring. The formula is disclosed as having high water solubility due to a preferred N-methoxy substituent. This substituent can also fragment to generate free radicals to initiate polymerization of an ethylenically unsaturated secondary polymer matrix, if present. The procedure used to formulate this pendant group requires even more steps than Ichimura et al., and grafting the pendant group to the polymer backbone still requires an aldehyde linkage to the phenyl ring.
A more universal way to make photosensitive polymers from poly(vinyl alcohol) and poly(vinyl pyridine) is needed which does not require final purification, is more flexible in the choice of starting materials, and can produce photosensitive polymers which are of similar photosensitivity whether prepared with a poly(vinyl alcohol) or poly(vinyl pyridine) backbone. In addition, a photosensitive composition is needed having reduced toxicity, increased stability, improved definition, and resistance to higher pH materials.